Obesity linked to our ability to digest carbohydrates

"I'm off the carbs" is a familiar refrain among dieters. But could this approach to losing weight be more beneficial to some people than others?

That's the implication of research suggesting that obesity may be linked to how our bodies digest the starch found in carbohydrate-rich foods like bread, rice and potatoes.

When we eat, an enzyme in saliva called salivary amylase kick-starts digestion by breaking down some of the starch found in carbohydrates into sugars. This enzyme is produced by the gene AMY1.

It's an unusual gene, in that people can have multiple copies of it, unlike most genes where there are just two. The more copies you have, the more enzyme you produce. One theory is that humans evolved to carry more copies of the gene as our diets shifted towards carbohydrate-rich foods.

Gene shortlist

Mario Falchi at Imperial College London and colleagues compared the genome sequences of a group of siblings where one was overweight, the other lean, and drew up a shortlist of genes which might help explain the difference. AMY1 topped the list.

Next, they studied a separate group of 5000 people from France and the UK and found that people with fewer than four copies of the gene were around eight times more likely to be obese than those with more than nine copies of the gene.

This suggests that people who are good at breaking down starch into sugars are less likely to be obese. However, why this should be the case isn't clear from this study as it doesn't take into account the amylase produced by the pancreas, says Falchi.

"It is possible that the effect of the salivary amylase genes isn't directly influencing the digestion of carbs and how much energy we get from them," he says. "It could be through more complex mechanisms such as influencing signalling pathways, or changing the gut microbiota."

Further studies to pull apart the mechanism are already underway.

Journal reference: Nature Genetics, DOI: 10.1038/ng.2939

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